A commercial or domestic refrigeration unit (e.g. a refrigerator or freezer) typically includes several electrical components which must be activated in a thermostatically controlled manner to provide refrigeration. These components typically include a compressor and electric fans. The compressor compresses the cooling media upon exit from the evaporator inside of the refrigeration unit, and delivers the compressed cooling media to a condenser outside of the refrigeration unit. The electric fans are typically positioned adjacent to the evaporator inside of the unit and adjacent to the condenser outside of the unit, to effect heat transfer to/from the evaporator and condenser to the air surrounding those components. Typically, power is provided to the compressor and fans by a thermostatic switch located inside of the refrigeration unit; in essence, the thermostatic switch closes when the inside temperature exceeds a threshold, causing power to be applied simultaneously to the compressor and fans. Power continues to be applied until the inside temperature reduces and the thermostatic switch opens, at which time the compressor and fans turn off.
Some modern refrigeration units include a fresh food compartment, which has a separately controlled temperature. The fresh food compartment is typically a drawer which when closed is isolated from the main compartment. To provide cooling to the fresh food compartment, the fresh food compartment is typically equipped with an active damper door, controlled by a damper motor. When open, the damper door permits cool air from the main refrigeration compartment into the fresh food compartment. (While the damper door is open, typically the evaporator fan is energized to move cool air from inside of the refrigeration unit into the fresh food compartment.) When the damper door is closed, the fresh food compartment is isolated from the main compartment, so that its temperature can be controlled separately from the main compartment.
In a typical refrigeration unit, the fresh food compartment is equipped with its own thermostatic switch to permit thermostatic control of the temperature of the fresh food compartment. This thermostatic switch detects when the temperature of the fresh food compartment exceeds a threshold, indicating that cool air from the main compartment must be introduced into the fresh food compartment. When the thermostatic switch detects this condition, the thermostatic switch changes state to its "hot" condition, in which it delivers electrical power to the damper motor to open the damper, and also delivers electrical power to the evaporator fan. When the fresh food compartment cools, the thermostatic switch again changes state to its "cool" condition, in which it delivers electrical power to the damper motor to close the damper, and ceases delivery of electrical power to the evaporator fan.
In a typical refrigeration unit, the fresh food compartment thermostatic switch is typically a dual-pole thermostatic switch. It appears that a dual-pole thermostatic switch has been used for this application, because power must be delivered to the damper motor to cause the damper motor to move (open or close) the damper door, both when the thermostatic switch changes state from its "cool" condition to its "hot" condition, and when the thermostatic switch changes back from its "hot" condition to its "cool" condition. A dual-pole thermostatic switch provides a ready means to provide power to the damper motor when the thermostatic switch is in both of its conditions.
Unfortunately, the use of a dual-pole thermostatic switch to control the fresh food compartment damper, substantially increases the cost of the refrigeration control circuit, and may also reduce the reliability of the circuit as a result of the greater number of contact points in a dual-pole thermostatic switch as compared to a single-pole thermostatic switch.